Page 4 - GSA Today December 2022

Biotic Enhancement of Weathering

over the Past 3.7 Billion Years

Gregory J. Retallack, Dept. of Earth Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA, gregr@uoregon.edu

ABSTRACT
Over the past four billion years, our sun
became 30% brighter, yet Earth’s water has
neither completely frozen nor boiled off
during that time. A theoretical solution to
this paradox is a carbon dioxide greenhouse
planetary thermostat regulated by evolu-
tionary advances in biologically mediated
silicate and apatite weathering. This carbon
sequestration history can now be quantified
using paleosols. Calculations of precipita- Figure 1. Stellar evolution and plan-
tion-normalized nutrient depletion rates etary temperature and atmospheric
(µmol mm a ) in paleosols ranging in age composition over the past 4.6 Ga,
–1
–1
back to 3.7 Ga show discrete order of mag- showing solar luminosity increase
and predicted temperature of Earth
nitude increases in carbon consumption by with current atmosphere or no
silicate and apatite weathering due to evolu- atmosphere (Ribas, 2009), envelope
of permitted temperatures from
tionary advances in life on land at around gypsum and life (Walker, 1982),
the Great Oxidation Event (2.45 Ga) and temperatures inferred from selected
paleosols (Retallack, 2013, 2018;
Neoproterozoic Oxidation Event (0.8 Ga). Retallack et al., 2016), and ice ages
This biological weathering countered (Walker, 1982).
increased solar luminosity and continued
emission of volcanic greenhouse gases.
INTRODUCTION
The faint young sun paradox arises from
stellar evolution of increased solar luminos-
ity through time (Ribas, 2009), which pre-
dicts frigid temperatures on early Earth,
with or without present atmosphere (Fig. 1).
However, moderate Archean temperatures
are inferred from salt stability, water-lain
sedimentary structures, and glacial episodes enhanced silicate and apatite weathering with increased productivity of terrestrial
(Walker, 1982). Paleotemperatures from prevented a terminal greenhouse (Retallack, vegetation (Retallack, 2022b). Paleosols are
paleosols (Fig. 1) are evidence of long-term 2022a). Consumption of CO by abiotic sili- not only evidence of carbon sequestration
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stability (Retallack, 2013, 2018; Retallack et cate weathering in lifeless Precambrian by silicate and apatite weathering, but also
al., 2016), averting terminal freezing, appar- landscapes was modeled by Rye and Holland include fossils as evidence of the evolution of
ent from Mars, as well as the other extreme (1998), but a role for life on land enhancing life on land.
of an uninhabitable inferno, apparent from weathering is indicated as far back as 3.7 Ga
Venus (Lovelock and Margulis, 1974). Both by paleosol salts, stable isotopic composi- PALEOSOLS AS PROXIES FOR
freezing and steaming may have been pre- tions, and phosphorus depletion (Retallack, CARBON SEQUESTRATION
vented by greenhouse gases such as CH and 2022b). Thus, theoretical concepts of biotic Paleosols are soils of the past, buried in
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CO regulated by the biological carbon cycle planetary temperature regulation can now be sedimentary or volcanic sequences, and
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(Schwartzmann, 2017). Continued volcanic assessed from the record of fossil soils back interpretable by comparison with modern
degassing of CO prevented a terminal ice- to 3.7 Ga. Generally declining atmospheric soils. Release of soluble alkali and alkaline
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house, whereas building of biomass and CO over time (Kasting, 2010) is not the only earth cations and bicarbonate into soil solu-
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consumption of carbonic acid by biotically issue involved, because soil CO increased tion by carbonic acid from CO in solution
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GSA Today, v. 32, https://doi.org/10.1130/GSATG543A.1. CC-BY-NC.
4 GSA TODAY | December 2022